Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: faster than speed of light?

Learn more physics!

Most recent answer: 04/12/2011
Can light be accelerated to a velocity faster than the speed of light? How? I've heard that in a vacuum, light can do such things but how does it work?
- Joseph Howerton (age 16)
Colorado Springs, CO, USA
No, light can't be accelerated to faster than c. Any blip of light traveling in a vacuum will move at that speed. In some other material, it will move slower.

Perhaps what you've heard about is that in some materials the "phase velocity" can be greater than c. The phase velocity is the velocity at which the pattern of electric field waves moves forward. You can think of that a a sort of set of stripes marking wavefronts within the blip of light. The stripe pattern can travel either faster or slower than the overall blip. However, the energy and momentum are just traveling at the blip speed. Furthermore, any information that the light carries is contained in deviations from the simple pattern. Those deviations travel at the blip speed, less than c. That blip speed is called the "group velocity".

If what you'd heard about is some other effect, follow-up.

Mike W.

(published on 02/26/2011)

Follow-Up #1: faster and slower light

There are many experiments showing dramatic increases and decreases to the speed of light like: Faster than normal light... Slower than normal light... and I have seen many more ways to get signals faster, like piling peaks onto peaks, using alternating types of coaxial wire in 3meter lengths (may be incorrect on those lengths), and probably others.
- harley (age 30)
Tempe, AZ
The slower-than-c light is not exactly news. Light travels at less than c in glass, water, alcohol, oil, diamond,.... Of course, Lene's experiments slowing light down to less than the speed of a thrown ball were pretty spectacular, but they aren't relevant to this discussion.

As for the faster-than-c hype, when it comes down to it the lead experimenter was quite careful in his description, as per the link you gave: "Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd. "The pulse of light is shaped like a hump with a peak and long leading and trailing edges. The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially 'reconstructs' the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber."

Let me repeat: no signal traveled faster than c.

Mike W.

(published on 04/12/2011)

Follow-up on this answer.